1. Field of the Invention
The present invention relates to a thermal protection structure suitable for forming a fairing for a rocket that flies through the earth""s atmosphere, undergoing aerodynamic heating, or for forming an airframe for a spacecraft, such as a self-contained space capsule for a spacecraft system that is launched into the exoatmosphere, reenters the earth""s atmosphere and is recovered.
2. Description of the Related Art
The outer surface of the airframe of a space vehicle, such as a rocket that flies through the atmosphere or a self-contained capsule that reenters the earth""s atmosphere, is exposed to an intense aerodynamic heating environment created by the impact and adiabatic compression of air molecules on the outer surface of the airframe. The outer surface of the airframe is covered with a thermal protection material to insulate the interior of the airframe from heat generated by aerodynamic heating. Such a thermal protecting material is called ablator. Typically, the ablator is a fiber-reinforced compound material, such as a carbon-fiber-reinforced plastic (CFRP) obtained by impregnating a reinforcing material formed of carbon fibers with a matrix resin, such as a phenolic resin. The CFRP is capable of withstanding heating at a high heating rate on the order of 15 MW/m2 which occurs when a space vehicle flying along a planetary orbit reenters the earth""s atmosphere.
The outer surface of the fairing of a space vehicle, such as a rocket or a reentry body, is subject to the detrimental action of raindrops and ice pellets causing erosion. Therefore the outer surface of the fairing must be resistant to such an erosive action and must be sufficiently resistant to aerodynamic heating. The fairing must be lightweight to increase the final velocity of the space vehicle. Therefore, it is desired to enhance the resistance of the fairing to aerodynamic heating without increasing the weight thereof. A thermal protection structure satisfying those requirements has been desired.
Accordingly, it is an object of the present invention to provide a lightweight thermal protection structure highly resistant to the erosive action of raindrops and the like and to aerodynamic heating.
According to one aspect of the present invention, a thermal protection structure comprises: a thermal protection layer formed by impregnating reinforcing fibers with a matrix resin; and a wear-resistant layer formed of a gas-pervious heat-resistant material and laminated on an outer surface to be exposed to heat of the thermal protection layer.
The wear-resistant layer covering the outer surface of the thermal protection layer prevents the direct impact of raindrops or ice pellets on the thermal protection layer, so that the reduction of the thermal protection ability of the thermal protection layer due to wear can be prevented. The gas-pervious wear-resistant layer permits the emission of gases produced by the thermal decomposition and carbonization of the thermal protection layer caused by aerodynamic heating into the environment.
Since the emission of the gases produced by thermal decomposition entails heat dissipation, cooling effect of heat transfer is available. The emission of the gases produced by thermal decomposition provides heat-insulating effect. The thermal decomposition of the thermal protection layer has endothermic effect. The endothermic effect, the cooling effect and the heat-insulating effect contribute to the improvement of the resistance to aerodynamic heating of the thermal protection structure. Since wear-resistant layer prevents the wear of the interior of the wear-resistant layer and the thermal protection layer, the thermal protection structure can ensure the high reliability of the fairing of the vehicle flying in the atmosphere.
Preferably, the thermal protection layer includes an outer fiber layer formed by impregnating the reinforcing fibers of short fibers extending in directions in a two-dimensional plane perpendicular to thickness of the thermal protection layer with a matrix resin.
The gases produced by the thermal decomposition of the matrix resin is able to permeate easily the thermal protection layer including the outer fiber layer of the reinforcing short fibers extending in directions in a two-dimensional plane perpendicular to the thickness of the thermal protection layer. The effect of heat absorption, heat dissipation and heat insulation by the gases produced by thermal decomposition provides the thermal protection structure with high thermal protection ability.
Preferably, the thermal protection layer further includes an inner fiber layer laminated on an inner surface of the outer fiber layer, the inner fiber layer being formed by impregnating the reinforcing fibers of long fibers which are longer than the short fibers with a matrix resin, the long fibers extending in directions in a two-dimensional plane perpendicular to thickness of the thermal protection layer.
The inner fiber layer formed on the inner surface of the outer fiber layer and including the reinforcing long fibers extending in directions in a two-dimensional plane enhances the mechanical strength of the thermal protection layer.
Preferably, the thermal protection structure further includes a heat-insulating layer covering an inner surface of the inner fiber layer and a metal coat layer covering an inner surface of the heat-insulating layer.
The heat-insulating layer covering the inner surface of the inner fiber layer further enhances heat-insulation to reduce thermal influence on equipment installed in the vehicle. The metal coat layer enhances the mechanical strength of the thermal protection structure and prevents the deformation and warping of the thermal protection structure due to thermal stress. The heat-insulating layer and the metal coat layer prevent the leakage of the gases produced by thermal decomposition into the vehicles. The metal coat layer changes the radiation rate of an inner surface facing an interior of the vehicle so as to improve thermal control. Thermal control reduces heat transfer from the high-temperature metal coat layer to the equipment installed in the vehicle by holding the radiation rate of the inner surface of the metal coat layer at a low level.
Preferably, the outer fiber layer is formed by randomly attaching prepreg pieces including the short fibers to an inner surface of a mold into a layer and hot-pressing the layer of the prepreg pieces.
Preferably, the inner fiber layer is formed by superposing prepregs including the long fibers in a plurality of layers an the inner surface of the outer fiber layer so that the long fibers in at least one layer of the prepregs extend in a direction different from a direction of the long fibers in another layer of the prepregs and heating the prepregs which are superposed.
Preferably, the thermal protection structure is used for forming a fairing of a rocket.
According to another aspect of the present invention, a thermal protection structure comprises: a base layer formed by impregnating reinforcing fibers with a matrix resin; and a surface layer formed of a porous material and laminated on an outer surface to be exposed to heat of the base layer.
Gases produced by the thermal decomposition of the base layer can be emitted through the surface layer of a porous material. The effect of heat absorption by the production of the gases, heat dissipation by the flow of the gases through the surface layer, and heat insulation by the gases emitted from the outer surface of the surface layer provides the thermal protection structure with high thermal protection ability.
Preferably, the thermal protection structure is used for forming an outer wall of a space vehicle which is designed to reenter an earth""s atmosphere from an outer space.